Dynamic crushing of sandwich panels with prismatic lattice cores

The dynamic out-of-plane compressive response of stainless steel corrugated and Y-frame sandwich cores have been investigated for impact velocities ranging from quasi-static to 200 ms⁻¹. Laboratory-scale sandwich cores of relative density 2.5% were manufactured and the stresses on the front and rear faces of the dynamically compressed sandwich cores were measured using a direct impact Kolsky bar. Direct observational evidence is provided for micro-inertial stabilisation of both topologies against elastic buckling at impact velocities below 30 ms⁻¹. At higher impact velocities, plastic waves within the core members result in the front face stresses increasing with increasing velocity while the rear face stresses remain approximately constant. While the finite element calculations predict the rear face stresses and dynamic deformation modes to reasonable accuracy, the relatively slow response time of the measurement apparatus results in poor agreement between the measured and predicted front face stresses. The finite element calculations also demonstrate that material strain-rate effects have a negligible effect upon the dynamic compressive response of laboratory-scale and full-scale sandwich cores.     

Theoretical prediction on corrugated sandwich panels under bending loads

In this paper, an aluminum corrugated sandwich panel with triangular core under bending loads was investigated. Firstly, the equivalent material parameters of the triangular corrugated core layer, which could be considered as an orthotropic panel, were obtained by using Castigliano’s theorem and equivalent homogeneous model. Secondly, contributions of the corrugated core layer and two face panels were both considered to compute the equivalent material parameters of the whole structure through the classical lamination theory, and these equivalent material parameters were compared with finite element analysis solutions. Then, based on the Mindlin orthotropic plate theory, this study obtain the closed-form solutions of the displacement for a corrugated sandwich panel under bending loads in specified boundary conditions, and parameters study and comparison by the finite element method were executed simultaneously.     

Multi-objective optimization of the sandwich panels with prismatic cores using genetic algorithms

In the present study, a new type of sandwich panels with prismatic cores, which are capable of load bearing as well as cooling, is optimized to have minimum weight and maximum heat transfer performance. In order to simultaneously minimize the total weight and maximize the heat transfer performance, a multi-objective optimization approach has been developed using genetic algorithms. A set of compromised solutions, known as the tradeoff surface, is obtained. The tradeoff information between the two objectives is exploited in terms of multi-functionality of the sandwich panels, and the relation between the two objectives is quantified in the present study. The detailed configurations and dimensions of the sandwich panels at the optima are provided. Some basic characteristics of the sandwich panels with prismatic cores have been observed in terms of their multi-functionality.     

Structural modeling of sandwich structures with lightweight cellular cores

An effective single layered finite element (FE) computational model is proposed to predict the structural behavior of lightweight sandwich panels having two dimensional (2D) prismatic or three dimensional (3D) truss cores. Three different types of cellular core topology are considered: pyramidal truss core (3D), Kagome truss core (3D) and corrugated core (2D), representing three kinds of material anisotropy: orthotropic, monoclinic and general anisotropic. A homogenization technique is developed to obtain the homogenized macroscopic stiffness properties of the cellular core. In comparison with the results obtained by using detailed FE model, the single layered computational model can give acceptable predictions for both the static and dynamic behaviors of orthotropic truss core sandwich panels. However, for non-orthotropic 3D truss cores, the predictions are not so well. For both static and dynamic behaviors of a 2D corrugated core sandwich panel, the predictions derived by the single layered computational model is generally acceptable when the size of the unit cell varies within a certain range, with the predictions for moderately strong or strong corrugated cores more accurate than those for weak cores. The project supported by the National Basic Research Program of China (2006CB601202), the National Natural Science Foundation of China (10328203, 10572111, 10572119, 10632060), the National 111 Project of China (B06024), the Program for New Century Excellent Talents in University (NCET-04-0958), the Open Foundation of State Key Laboratory of Structural Analysis of Industrial Equipment, and the Doctorate Foundation of Northwestern Polytechnical University.     

Mechanical behavior of sandwich panels with tetrahedral and Kagome truss cores fabricated from wires

Wires are great candidates as the raw material for truss periodic cellular metals because they can display high strength as in piano wires, are easy to fabricate, and can be controlled to be defect free. New approaches based on tri-axial weaving of wires to create ideal trusses, i.e., tetrahedral and Kagome truss have been presented. The mechanical properties of the sandwich panels with the truss cores fabricated by using the new approaches under compression and bending loadings are analyzed by elementary beam theory and experiments. The relative density, stiffness, and strength of the sandwich panels are estimated by the derived equations and compared with the measured results. The failure mechanisms of the sandwich panels are analyzed, and also benefits and shortcomings of each approach with respect to mechanical performance and production are discussed.     

爆炸载荷下分层梯度泡沫铝夹芯板的失效模式与抗冲击性能

采用弹道冲击摆系统开展了爆炸载荷下分层梯度泡沫铝夹芯板的变形/失效模式和抗冲击性能实验研究,并配合激光位移传感器得到试件后面板中心点的挠度-时程响应曲线。研究了炸药当量和芯层组合方式对夹芯板试件变形/失效模式和抗冲击性能的影响。实验结果表明,泡沫铝夹芯板的变形/失效模式主要表现为面板的非弹性大变形,芯层压缩变形、芯层拉伸断裂以及芯层剪切失效。在研究爆炸冲量范围内,非梯度芯层夹芯板的抗冲击性能明显优越于所有分层梯度芯层夹芯板。对于分层梯度夹芯板试件,爆炸冲量较小时芯层组合形式对分层梯度芯层夹芯板的抗冲击性能的影响不大,而爆炸冲量较大时,最大相对密度芯层靠近前面板组合形式的分层梯度夹芯板试件抗冲击性能较好。研究结果可为泡沫金属夹芯结构的优化设计提供参考。     

Equivalent models of corrugated panels

The design of corrugated panels has wide application in engineering. For example corrugated panels are often used in roof structures in civil engineering. More recently corrugated laminates have been suggested as a good solution for morphing aircraft skins due to their extremely anisotropic behaviour. The optimal design of these structures requires simple models of the panels or skins that may be incorporated into multi-disciplinary system models. Thus equivalent material models are required that retain the dependence on the geometric parameters of the corrugated skins or panels. An homogenisation-based analytical model, which could be used for any corrugation shape, is suggested in this paper. This method is based on a simplified geometry for a unit-cell and the stiffness properties of original sheet. This paper outlines such a modelling strategy, gives explicit expressions to calculate the equivalent material properties, and demonstrates the performance of the approach using two popular corrugation shapes.     

Effect of temperature on the compressive behavior of carbon fiber composite pyramidal truss cores sandwich panels with reinforced frames

This paper focuses on the effect of temperature on the out-of-plane compressive properties and failure mechanism of carbon fiber/epoxy composite pyramidal truss cores sandwich panels(CF/CPTSP). CF/CPTSP with novel reinforced frames are manufactured by the water jet cutting and interlocking assembly method in this paper. The theoretical analysis is presented to predict the out-of-plane compressive stiffness and strength of CF/CPTSP at different ambient temperatures. The tests of composite sandwich panels are performed throughout the temperature range from -90℃ to 180℃. Good agreement is found between theoretical predictions and experimental measurements. Experimental results indicate that the low temperature increases the compressive stiffness and strength of CF/CPTSP. However, the high temperature causes the degradation of the compressive stiffness and strength. Meanwhile, the effects of temperature on the failure mode of composite sandwich panels are also observed.     

Influence of imperfections on the performance of metal foam core sandwich panels

Sandwich panels and beams are used in bending and compression dominated components. The retention of their load capacity in the presence of imperfections is a central consideration. To address this issue, sandwich beams with metallic foam cores have been tested in four-point bending following the introduction of imperfections, created by impressing the face sheets. Limit load expressions for face yielding, core shear, and indentation failure have been developed and used to construct failure mechanism maps. From these maps, specimen designs were determined. Imperfections were introduced by indenting to varying penetrations. The indents were located on both the compressive and tensile side of bending configurations. Experimental measurements of the load/deflection response are obtained and compared with finite element results.     

Compressive strength of edge-loaded corrugated board panels

Postbuckling strength of simply supported corrugated board panels subjected to edge compressive loading has been studied experimentally using a specially developed test fixture. Although the load versus out-of-plane displacement response was highly sensitive to the presence of initial imperfections in the panels, the collapse loads did not vary much, which is attributed to the stable postbuckling behavior of the plates. Thin plates collapsed at nearly twice the buckling load, while thick panels collapsed at loads below the elastic critical buckling load. Local buckling of the facing on the concave side of the buckled plate was observed at load levels close to the collapse load. The plate collapse was triggered by compressive failure of the facings that initiated at the unloaded edges. A simplified design analysis was derived based on approximate postbuckling analysis and compared with an existing design formula for corrugated board panels and boxes.     

The blast resistance of sandwich composites with stepwise graded cores

Shock tube experiments were performed to study the dynamic response of sandwich panels with E-Glass Vinyl Ester (EVE) composite face sheets and stepwise graded styrene foam cores. Two types of core configurations, with identical areal density, were subjected to the shock wave loading. The core layers were arranged according to the density of the respective foam; configuration 1 consisted of low/middle/high density foams and configuration 2 consisted of middle/low/high density foams. The method to calculate the incident and reflected energies of the shock wave, as well as the deformation energy of the specimen, were proposed based on the shock wave pressure profiles and the high speed deflection images that were obtained. The experimental results showed that configuration 1 outperformed configuration 2 in regards to their blast resistance. Significant core material compression was observed in configuration 1, while in configuration 2 the core layers disintegrated and the front skin (blast side) fractured into two pieces along the midsection. The estimated energies were then calculated for both configurations. The total energy difference between the incident and reflected energies was almost identical, even though the deformation energy for configuration 2 was larger.     

Design of metallic textile core sandwich panels

Metallic sandwich panels with textile cores have been analyzed subject to combined bending and shear and then designed for minimum weight. Basic results for the weight benefits relative to solid plates are presented, with emphasis on restricted optimizations that assure robustness (non-catastrophic failure) and acceptable thinness. Select numerical simulations are used to check the analytical results and to explore the role of strain hardening beyond failure initiation. Comparisons are made with competing concepts, especially honeycomb and truss core systems. It is demonstrated that all three systems have essentially equivalent performance. The influence on the design of a concentrated compressive stress that might crush the core has been explored and found to produce relatively small effect over the stress range of practical interest. “Angle ply” cores with members in the ±45° orientation are found to be near optimal for all combinations of bending, shear and compression.     

Edge-compression fixture for buckling studies of corrugated board panels

A test fixture, developed for evaluating the preand postbuckling response of simply supported, nearly flat, rectangular corrugated board panels subjected to edge compression is evaluated. The test fixture enables loading of panels into the postbuckling regime until collapse. The shadowmoiré method verified that buckling in the first mode occurred, and that there was symmetry of the adge-boundary conditions. Through an iterative regression model, experimental curves of load versus out-of-plane displacement for isotropic panels were fitted to an equation governing the nonlinear postbuckling response. This method provides the critical buckling load, a postbuckling parameter and the amplitude of initial imperfection of the panel. Comparison with analytical results revealed that simply supported boundary conditions were closely achieved. Examination of compressively loaded corrugated board panels showed that collapse occurred due to compressive failures of the facings in the highly stressed edge regions without severe influence from stress concentrations at load introduction and edge supports.     

Minimum weights of pressurized hollow sandwich cylinders with ultralight cellular cores

Long, open-ended, hollow sandwich cylinders with ultralightweight cellular cores are optimized under uniform internal pressure for minimum weight design. Five different core topologies are considered: Kagomé truss, single-layered pyramidal truss, double-layered pyramidal truss, single-layered corrugated core and double-layered corrugated core. The highly porous cellular materials are homogenized to obtain effective constitutive relations. Close-formed solutions are presented for the forces and stresses in individual structural members of the sandwich, which are then validated by finite element calculations. Optimization of the sandwich-walled hollow cylinder is achieved using a quadratic optimizer, subjected to the constraints that none of the following failure modes occurs: facesheet yielding; facesheet punch shearing (active only for truss-cored sandwiches); core member buckling; core member yielding. In comparison with hollow cylinders having solid walls, truss-core sandwich cylinders and single-layer corrugated core sandwich cylinders are found to have superior weight advantages, especially for more heavily loaded cases. With the consideration of both weight efficiency and failure modes, sandwich-walled hollow cylinders having Kagomé truss core with pyramidal sub-geometry have the best overall performance in comparison with other core topologies.     

金属正交波纹夹芯结构的动态压缩响应

通过理论和数值方法,对冲击载荷下金属正交波纹夹芯结构的动态压缩响应进行了研究。考虑材料应变率影响,建立了金属正交波纹夹芯结构动态响应的理论模型,同时对它的动态压缩响应进行了有限元模拟。结果表明,考虑材料应变率影响的理论模型的预测结果与有限元模拟结果吻合较好。进一步对多层正交波纹夹芯结构的动态压缩响应进行了数值模拟,获得了不同速度冲击下的变形模式,分析了层数对其动态响应的影响。研究发现,通过增加层数能够有效地增强结构的缓冲吸能能力,但层数超过4层以后增强效果不明显。

     

仿生波纹夹层结构耐撞性分析及优化

为提高薄壁夹层结构耐撞性,以虾螯为仿生原型,设计梯度分布的仿生波纹形夹层结构,包括单层、双层和三层波纹结构。以初始峰值载荷F_p、比吸能E_s为耐撞性指标,利用有限元法分析了单元高宽比γ（γ₁、γ₂和γ₃分别为单元第1层、第2层和第3层的高宽比）对波纹夹层结构耐撞性的影响,采用多目标粒子群优化方法得到了夹层结构最优参数。结果表明,单层波纹结构耐撞性随单元高宽比γ的增大逐渐变差,双层波纹结构下层结构单元高宽比γ对耐撞性的影响大于上层结构单元高宽比γ对耐撞性的影响,较小的γ值有利于提高三层波纹结构的比吸能。结构优化结果表明:单层结构最优尺寸γ₁为0.8;双层结构最优尺寸为γ₁ = 0.5和γ₂ = 1.2;三层结构最优组合为γ₁ = 0.6,γ₂ = 0.6和γ₃ = 0.9。上述结果可为薄壁夹层结构轻量化设计提供新思路。     

A plastic indentation model for sandwich beams with metallic foam cores

Light weight high performance sandwich composite structures have been used extensively in various load bearing applications.Experiments have shown that the indentation significantly reduces the load bearing capacity of sandwiched beams.In this paper,the indentation behavior of foam core sandwich beams without considering the globally axial and flexural deformation was analyzed using the principle of virtual velocities.A concisely theoretical solution of loading capacity and denting profile was presented.The denting load was found to be proportional to the square root of the denting depth.A finite element model was established to verify the prediction of the model.The load-indentation curves and the profiles of the dented zone predicted by theoretical model and numerical simulation are in good agreement.     

Theoretical model for sound transmission through finite sandwich structures with corrugated core

Due to the promising applications of lightweight double-leaf structures in noise control engineering, numerous investigations have been performed to study the vibroacoustic properties of these structures. However, no attention has been focused on the vibroacoustic properties of finite double-leaf structures with corrugated core used extensively in constructing the hulls of bullet passenger trains. In the present paper, a theoretical model is developed to predict the sound transmission loss (STL) characteristics of simply supported double-leaf partitions with corrugated core. The boundary conditions are accounted for by writing the displacements of the face plates in a series form of modal functions. The model predictions are validated by comparing with existing experimental measurements. The vibroacoustic properties of the sandwich construction are examined and the physical mechanisms for sound transmission through the structure explored, including the phenomena of ‘coincidence resonance’ and ‘standing wave resonances’. The effects of structural links, structural dimensions, inclination angle of the corrugated core, as well as the thickness of face plates and core layer on the STL are systematically investigated.